PROCESSING – REVERB Part 1

The ubiquitous Lexicon 224XL reverb, modeled here in software for the UAD-2 platform.

Reverberation is a naturally occurring phenomenon that happens all around us, all the time. It’s the pattern of reflections that builds up in a space when direct sound waves bounce off obstacles and reflect back in different directions in different length paths of travel.Over time, this web becomes pretty complex, although it also diminishes over time as sound loses energy from the multiple collisions involved.

Reverberation is a mouthful to say, so we often shorten it to reverb in polite conversation, and sometimes just plain ol’ ‘verb.

I tend to say say reverb. I think I know her well enough after all these years to call her by her first name.

REVERB IN GENERAL

Our brains use a number of spatial cues to interpret our environment. Where exactly is that elephant stampede rapidly approaching from?

Thankfully, we have two ears, and they’re located on opposite sides of our head, which makes it possible for our brains to estimate the direction and distance of the source of sounds we can hear around us.

When sounds arrive at our ears, we examine both inputs and compare them virtually instantaneously, in order to determine the sound’s point of origin.

It’s not as simple as how loud a sound is, since tonality of sound changes as it moves through air and/or other materials due to energy loss being different at different frequencies.

The highs fall away with distance, so far away things will sound muffled and dark relative to nearby sounds.

The distant sounds have a lower amplitude as well, all things being equal. So, things are quieter and darker when they are further away, which we know instinctively to be true, and reverb tends towards these tonalities to be effective.

So far, I’m just talking about the direct sound reaching one or both of our ears from the source in a straight line.

Then we have the added complication of room sound. This is raw reverb, at least the indoors variety where there are several nearby boundaries or surfaces, usually somewhat reflective. Walls, floor, ceiling, bookcase shelf full of books, TV screen, curtains, paintings.

Whatever is in the space, it all counts towards making the room sound the way it does.

Room sound colours sounds in a complex way. Go through the rooms of your home, clapping your hands and seeing what happens.

If you get a complex series of flutter echoes, you will know that is what will be added to a signal you record at that spot with a microphone.

It’s been a challenge to find ways to generate and add reverb artificially. Over the last seventy odd years, all sorts of progress has been made.

There are a whole bunch of reverb types to choose from nowadays. They have varying properties and parameters and preferred applications abound.

There are springs and plates, digital algorithmic reverbs and digital convolution reverbs. There are presets for halls, rooms, chambers, springs, plates, non-linear reverbs and even weird and wonderful places from famous locations like Wembley Stadium or the Taj Mahal, or movie-friendly foley locations like warehouses, closets, tiled swimming pools or the interior of a specific automobile.

Do you want the vehicle’s window rolled up or rolled down for the reverb you apply to the dialog spoken inside the car? That’s the sort of thing you can choose with the top end reverbs like the modern Lexicon 960LD, a surround-sound-capable 8-channel hardware algorithmic reverb. It takes some pretty intense calculations to get these kinds of results, which is why the very best of these algorithmic reverbs can cost well into five figures.

Fortunately, convolution reverbs sound even more realistic, and are a great deal cheaper, although rather limited in some important respects. If the reverb works, great, but if not, it’s not too tweak-able. The reverb decay time parameter that is so easily changed in algorithmic reverbs is a problem for convolution reverbs.

Since the Wet/Dry balance control and the Reverb Decay Time parameters are the major influence on shaping the reverb you want, this makes using a convolution reverb one of those “maybe it will work, or maybe it won’t” affairs for any particular application. When it’s underlying IR (impulse response, more on that below) compliments the mix, it provides some very natural reverb.

Quantum Leap’s Spaces, UAD-2 DreamVerb, UAD-2 Lexicon 224XL, Sonnox Oxford Reverb, and Logic’s Space Designer are just a few of the many high quality reverb plug-ins, each with a different sound, some convolution, some algorithmic.

There are lots to try, but do check the system requirements before buying anything. Be aware that plug-in costs add up quickly, so beware you buy only what you really need and make it a quality plug-in.

UAD plug-ins only run if you have at least one of their PCI-e hardware cards installed in your computer, and many of the best UAD plug-ins are optional extras at greater cost, in many cases well into three figures each to buy. Try demos first.

I will say that the UAD range is fantastic, full of interesting things, and really excellent value, especially compared to the cost of the hardware they emulate, and I use them all the time myself. It’s still rather expensive to stock up on these things though.

The Waves range is really expensive to collect too, but they have many low-cost ones that are great value. They do not require you to buy a PCI-e card, like UAD, but obviously there are system requirements you must check before buying Waves plug-ins.

WHY ADD REVERB?

Using a reverb will help you blend disconnected sounds back into the mix.

Using a reverb lets you adjust the front/back depth perspective in your mix.

Using a reverb can make things appear fatter and thicker.

Using a reverb, you can spread a mono ‘point source’ sound out across the stereo image over a wider area.

Using a reverb, you can add sustain by adding complex reflections and a so-called reverb tail.

Using a reverb, you can enhance aesthetic properties generally.

Using a reverb, you can make something sound as if it was recorded in a larger space than it actually was.

The main thing to remember with adding reverb is that less is more. It’s easy to add, but virtually impossible to remove, like salt in food.

Secondly, it is extremely unlikely that you will want to insert reverb across the insert points of a real-world mixing console.

In that situation, you would use an available console Auxiliary Send stereo buss to route signals to the reverb’s Left and Right inputs, and return the signals using either the Auxiliary Return channels of the mixer or perhaps spare channel inputs or even group inputs if no channels are available.

Normally, in a DAW, you would insert reverb plug-ins on an Auxiliary Input type of track, and use the Sends on your individual channels to send more or less of the signal to the buss that feeds the reverb track you just created. With this approach, you can send many channels to the same reverb track, by using a send on each track you want to input to the reverb.

The various channel’s Sends will all have an individual Send level control so that you can balance the mix of signals you are sending into the reverb inputs, one against the other.

PRE-FADER SENDS AND POST-FADER SENDS

It is very important to note the difference between Pre-Fader sends and Post-Fader sends.

With a Post-Fader send, you can lower the channel in the mix and the applied reverb will lower right along with it, in proper proportion.

If you use a Pre-Fader Send instead, the reverb level in the mix for that sound will stay the same even though you lower the level of the channel fader for the original sound, making the sound in question appear wetter and wetter until it is completely disembodied in a cloud of reverberation. This sounds very cool on a sunny day in Kingston, Jamaica for a dreamy reggae track, but may be a lot less popular in your tight and dark speed metal riff-fest.

REVERB CHAMBERS AND GETTING REAL ROOM SOUNDS INTO YOUR MICROPHONES

Back in the day, studios would build a special room for reverb, and put a speaker at one end and a microphone at the other. The mic would pick up the sound of the space as well as the sound from the speaker, adding a bunch of interesting reflections to the sound.

Usually, the rooms were concrete, undecorated chambers, to maximize the amount of reflected sound reaching the microphone. They called them reverb chambers. They were great, but they required an awful lot of space to implement. Only big studios could really afford to do this.

The studios at the iconic Capitol Records building in Los Angeles in the USA are famous for a particular reverb chamber that was used on Frank Sinatra and Nat King Cole and countless other icons of popular music in the 20th Century. Many studios had these when they were the only option available.

Room sound is a term for the astonishingly complex web of reflection paths that sound will take once a wave is sent forth from a source, bouncing around a room until it’s energy is spent.

All of the materials found in the room or space will have differing sound absorption characteristics, impacting the music significantly, so that the reverberant field will change depending on how much wood or glass or upholstery you have near to your sound source.

Good engineers will use different materials deliberately to get the kinds of sounds they are after. Sheets of plywood and heavy blankets are both useful for temporarily enhancing the acoustic properties of the areas around a microphone.

Don’t be shy with this. It’s a very helpful technique to modify the acoustics of the space you record in on an ad hoc basis. The results can be excellent.

For instance, brass sounds great when it is bouncing off a sheet of plywood at a distance of ten or fifteen feet. It also helps to have hardwood floors, and if you do, you can use a spare mic to pick up reflections from a few inches above the wood of the floor while they are playing. Blend it in with the direct mic to add a little of the wooden room sound that brings the brass alive in the mix with a complex sense of space.

In a similar vein, put a quilt or blanket behind the vocalist’s mic if you can, to stop the singing reflecting back into the mic from a nearby wall on the far side of the mic.

This would reduce room sound entering the microphone, dramatically in most cases. It works by dampening down or removing the midrange and high frequencies that would otherwise get into the microphone after bouncing off the nearest boundary (the wall the singer faces).

The sound is drier, and more isolated, as well as darker and a little more claustrophobic than before.

This kind of dry separation is exceptionally useful when it’s mixing time, since it’s usually very easy to ‘wet’ down a vocal with more reverb or delay, but it’s extremely hard to remove unwanted reverb and/or delay and/or room sound.

UAD-2 RealVerb-Pro software reverb emulation that clearly reminds us of the influence of materials (due to their acoustic properties) on the character of the reverb

So, a great way to add reverb is to capture genuine room sound by appropriate microphone positioning in an actual room (or other environment, almost always trying to do so at the time the instrument or singer is performing – but not necessarily.

We can do this while the sound we are adding it to is being generated (say, a drummer playing) or, at a pinch, played back through full-range speakers.

I have added drum room sound very successfully to a jazz trio recording after the fact by playing only the drum tracks back in a room and miking the result.

You need a high-quality, full-range, dry mix of the drums, possibly mono if you intend to use only one room mic. This way, you can play that mono mix into the miked room, and then you can pan the original hard left and the room mic hard right in your mix. you’ll get a very realistic sense of space.

Be aware that mono phase checks are necessary to be sure you are not losing low end in the kit when the stereo mix is collapsed to mono.

Play it back through full-range studio monitors placed on stands in the room at the location where the drumkit performance occurred.

Ideally, use a decent 2.1 system (stereo speakers with a third speaker for the sub-bass frequencies). Capture the sound in the room by placing room mics as desired.

Typically, you would put mics up high in the air on the far side of the room, or high up some way behind the kit, maybe in a corner to get a sense of even more complex reflections. Two mics give you a spread you could use in the stereo image, in various ways.

Mono room sound is also really useful, since you can capture it with a single microphone and mono-panpot position it in the image, perhaps hard-panned to one side opposite the original sound.

It’s also very handy to pan the mono room sound to exactly the same position in the stereo image as the original mono source. This is very popular on electric guitar parts, for example.

We can process and interpret sounds occurring throughout an incredible dynamic range. We can take in all kinds of sounds through our ears ranging from a pin drop to a screaming jet engine. That’s a truly colossal range, when you consider that the decibel scale is logarithmic and every 6dB increase is a doubling in loudness! Holy thunderclap, Batman!

Recording at home in various rooms means you are generating soundwaves in your home, which will give them certain characteristics that may or may not be flattering or appropriate when they enter the microphone.

Guitar amps pointing into a tiled bathroom through an open door (IMPORTANT: ALWAYS avoid contact between water and amplifiers!) will give you a reverberant sound that might rock an epic solo if the pipes aren’t running at the time. The same amp sounds totally different when it’s moved into the funky shag carpeted living room and pointed at a large plush couch that sits in front of a heavy pair of drawn velvet curtains.

The more you move the mic away from the source, the more the ratio of room sound to direct sound increases. Experiment for yourself and see!

Let’s look now at the spring reverb. Sproinggggg!

SPRING REVERB

AIR Spring Reverb

To emulate room sounds, many a guitar amp has featured a spring reverb. This is literally a length of metal spring in a little box. The guitarist’s best friend for many a year.

It’s a low-budget, lo-fi kind of a sound, and adds loads of character to a mix, but it’s also a bit trashy and certainly not suitable for a main vocal reverb except as a deliberate novelty effect.

I love this sound, and it will add a nice textural feel to a guitar part, so don’t write it off just because it is lo-fi and possibly noisy and strongly metallic in nature. Those can be admirable qualities in many genres.

PLATE REVERB

This is a really, really useful form of reverb that will almost certainly find a place in 95% of your mixes.

Plate reverb sounds awesome on snare drums and lead vocals, especially on ballads. It’s a lush sound, with rich resonant overtones and it can add a subtle edge of metallic ringing to sustained sources.

The reverb tail is rich and complex, and sounds great. There’s a late-night jazz and blues feeling about some plate sounds, and you’ll turn to them regularly for their warmth and evocative sounds.

UAD-2 emulation of the classic vintage EMT 140 Plate reverb

Particularly great plate emulations are available from Universal Audio for their UAD-2 platform, which runs on a PCI-e card that you install into a spare full-length PCI-e slot in your Mac or PC computer.

Pictured above is their UAD-2 emulation of the first successful commercial plate found in many studios, the EMT 140, an actual electro-mechanical plate, being a very big sheet of metal with electric terminals hooked up to either end of it, and suspended tightly on springs so that it could freely vibrate.

A later type, the EMT 250, employed digital methods to simulate plates rather than being an actual physical plate, and this too is represented in the UAD-2 selection of excellent plug-ins.

I can’t recommend the Universal Audio UAD-2 range of plug-ins highly enough, although they are rather expensive to begin collecting.

You need the PCI-e card to run them, so that adds a fair cost to getting into the game with them, I’m sorry to say. But, like everything Universal Audio sells, they rock big time and are worth every penny for the results you get.

ALGORITHMIC REVERB

After physical plates had been around for a couple of decades or so, computers were brought to bear on the complex math of simulating reverb fields.

The amount of computing power was so high for the time that a dedicated, mainframe computer was required for reverb algorithms to be processed quickly enough to respond to an input signal appropriately.

Devices like the Quantec Room Simulator and the AMS Reverb soon came into their own, and the Lexicon 224XL and then it’s offspring the Lexicon 480L sat at the top of the tree after a while as the most popular reverb in the larger studios’ control rooms.

These machines were using powerful mathematical algorithms to create and apply artificial reverb.

Recordings started to be dominated by the reverb of the successor to the 224XL, the 480L, and it became very cool to gate the tail of the reverb.

This was partly due to the newly discovered ease of gating signals brought by the advent of the SSL console, with a compressor and noise gate available on every one of it’s input channels. Phil Collins and Peter Gabriel and others made the most of the new reverbs.

Huge hair band power ballads filled with bloated reverb became immensely popular throughout the world. The downside of the algorithmsl, though, was that they were artificial and alwasy remained artificial in sound. More realism appeared out of reach for technology of the day.

Eventually, the algorithmic claim to the throne was challenged by a new upstart, the convolution reverb, which appeared in the mid-1990’s in the mass marketplace.

CONVOLUTION REVERB

Convolution reverbs work on a very different principle than other reverb effects.

The pattern of reflections that will build up in a real space are uniquely characteristic to that space, being affected by everything from building materials to air temperature and pressure variations within the space.

The idea with convolution is to make a short, sharp sound with a clear transient and a minimal decay time, much like the gunshot from a starter pistol.

The impulse response of the room is measured and stored as an “impulse response” file in the software plug-in, as part of a library of real spaces.

Impulse responses (IRs) for various spaces are included with convolution reverb plug-ins.

You can also make your own IR recordings, in order to capture and use the reverb characteristics of a space yourself, such as your bathroom or your concrete-floored double garage or even your local church.

The realism of convolution means that they sound more convincingly real than algorithmic reverbs, but the downside is that convolution relies on the IR, and the effect cannot be tweaked with the same ease and freedom as you enjoy when altering algorithmic reverb settings.

Algorithmic reverbs are more flexible than convolution reverbs, and sound less real, but convolution is a remarkably lifelike sound when the space fits the music like a glove. If none of the IRs are satisfactory, then you will need to abandon the convolution approach and use another method.

VERY SHORT DELAYS

It would be remiss of me not to mention that many records get made without ever using a reverb.

Some of them only contain artificially generated synthetic sounds that have no natural space around them at the time of recording because they are entirely synthesized. Of these, some are instrumentals and all feature some instrumental sections where no vocals are present.

In order to provide depth to the image in these situations, and not use reverb, you would want to turn to very short delays.

In the real world, there is always a time delay between a sound being emitted and it reaching the nearest boundary (wall, floor, ceiling, table, bookcase, etc). This is the pre-delay parameter of a reverb, which governs the length of time before the onset of reverberation begins.

Pre-delay is set in milliseconds, and the value chosen represents the time it takes to reach that first boundary and the onset of reverberant reflections beginning to build up in the space.

We know the speed of sound in air from the laws of physics. Assuming standard air temperature and pressure at sea level, it’s roughly a millisecond per foot.

That means a pre-delay of 20ms would represent the nearest boundary being 20 feet from the source.

That is quite a large space. Make it 6ms and it’s a very small room.

Add to this the fact that we humans have serious trouble detecting two sound events as separate sounds if they arrive at our ears within 30ms of each other. Under that value, sounds blur together, and cues are lost that we rely upon for critical listening.

This is why the other time-based effects (the modulation effects) all use very short delays (chorus is 1ms to 10ms, for example).

These short delays are really good at thickening and multiplying sound, without making the listeners feel that the additional layers of sound are unconnected from the original signal.

They also take up less headroom and sonic space in the mix than your average reverb, since they are much less cluttered and do not have the long sustained tails that reverb provides (when it is not being artificially gated before the tail ends).

Snare Trap preset shown on the WAVES TrueVerb reverb emulation

Reverb can mask sounds, but it can also help to unmask them.

There will be lots of controls to manage the envelope characteristics of the reverb as it builds and decays over time, and filters to shape the tone as well.

Real reverb loses highs with distance, so you would roll off treble frequencies from your reverb returns if you are interested in realism, perhaps for jazz, classical or folk.

A key advantage of gating reverbs is that the clutter and masking effects of reverb are removed with the gate’s closing, and this means the reverb can be louder and beefier than would be possible if it were allowed to sustain for a long time after each snare strike or vocal grunt.

If you prefer not to have the gated sound, even though it can be far more subtle than a Phil Collins record might make you think, then you will want to use the short delays to fill in sounds and add sonic texture and depth. They will also help with size and blending mix tasks alongside their obvious role in fattening sounds a little with a cluster of short, quiet, spread-out delays. Mix density is kept lower than it would be if you had used a sustain reverb instead of delays.

The trick is to keep the delays short enough that they don’t sound like delays. We’ll discuss the so-called Haas effect and other short-delay-specific psychoacoustic techniques when i get to the mixing series in late June.

In the next blog, I’ll discuss the various reverb parameters, including pre-delay, type of reverb, envelope controls and filters. There are a fair number of parameters, but all the same, it will be a shorter blogpost than I have posted today. See you then!